Concentrated solar power (CSP) involves the use of lenses, mirrors or other optical apparatus to focus solar radiation from a large incident area onto a small area. The energy from the solar radiation is then used to generate electrical power. Concentrated solar power has the potential to become an important energy source in the future.
There have been many proposals for concentrated solar power technology. The technology believed to have the most potential for providing high efficiency power generation is the central receiver solar thermal power plant. This technology involves the use of a solar radiation receiver, mounted atop a tower, to receive solar radiation that is reflected to be incident upon it by an array of tracking reflectors located in a solar field around the tower. The tracking reflectors are typically heliostats and the array of heliostats is commonly referred to as a heliostat field.
FIG. 1 is a diagrammatic illustration of a conventional direct steam concentrated solar thermal power plant in which solar radiation is reflected by a heliostat field 2 to be incident upon a solar radiation receiver 4 mounted atop a tower 6. The reflected solar radiation directly heats water circulating in a power generation circuit 8. This generates superheated steam which is used to drive a steam turbine generator set 10, and thereby generate electrical power, in a well-known manner using the Rankine cycle. In addition to the steam turbine generator set 10, the power generation circuit 8 includes an air-cooled condenser 12 and a feed water heater 14.
Direct steam concentrated solar thermal power plants can only operate effectively during daylight hours at times when the available solar radiation reflected to be incident upon the solar radiation receiver 4 is sufficient to generate superheated steam at the required pressure and temperature in the power generation circuit 8. This is because the high pressure and high temperature steam cannot be stored easily for later use.
In order to overcome this drawback, direct steam concentrated solar thermal power plants with energy storage capability have been proposed. These plants use high specific heat capacity thermal energy storage fluid, typically a molten salt or a mixture of different molten salts, for energy storage. Thermal energy is stored during a charging cycle by heating the molten salt and the thermal energy is subsequently recovered during a discharging cycle to heat water, and thereby generate steam, in a power generation circuit. The generated steam is then used to drive a steam turbine generator set to generate electrical power.
A first heat exchanger is needed to transfer thermal energy from steam not used to drive the steam turbine to the molten salt during the charging cycle. A second heat exchanger is also needed to recover thermal energy from the hot molten salt, and transfer it to the power generation circuit, during the discharging cycle. There are, therefore, multiple heat transfer stages, which reduces the efficiency of this type of solar thermal power plant. In particular, as the steam cools during the charging cycle, it undergoes a phase change whereas the molten salt does not. The amount of thermal energy transfer to the molten salt is, thus, limited thereby limiting the maximum attainable temperature of the molten salt and resulting in what is known as a ‘pinch point loss’. Consequently, when thermal energy is recovered from the hot molten salt to generate steam in the power generation circuit, the generated steam attains a significantly lower temperature and pressure than the steam originally used to heat the molten salt. This significantly reduces the efficiency of this type of solar thermal power plant. Furthermore, the lower steam pressure in the power generation circuit may be insufficient to run the steam turbine generator set at full load, meaning that the power generation requirement cannot be met.
Another type of concentrated solar thermal power plant utilises a solar radiation receiver to directly heat a high specific heat capacity thermal energy storage fluid such as molten salt. In this type of plant, thermal energy is recovered from the molten salt to heat water in a power generation circuit, and thereby generate steam to drive a steam turbine generator set, irrespective of the prevailing daylight conditions. This type of solar thermal power plant is generally less efficient than a direct steam concentrated solar thermal power plant during daylight hours because the steam in the power generation circuit is generated at all times through indirect heating, by recovering thermal energy from the molten salt in a heat exchanger. Furthermore, this type of solar thermal power plant is generally less attractive than a direct steam concentrated solar thermal power plant because its construction is more complex (and hence more costly) and the technology is still at a relatively early stage of development.
It would, therefore, be desirable to provide a solar thermal power plant having improved efficiency and operational flexibility.